5 Answers
5

I think people are being misled by overly literal interpretations. Let's first note this definition from Oxford Dictionaries (it's also the one automatically shown by Googling define desert island)...

desert island- a remote tropical island, typically an uninhabited one
"Carwyn seemed like a castaway on a desert island"

I would say that word "typically" is crucial, in that it implies a desert island might not in fact be uninhabited. Particularly since the usage example implies at least one inhabitant (the castaway lives there, even if he hopes his residence will be temporary).

More importantly, I can find anynumberofonlinedefinitions for desert island (that's 5 just there). To all intents and purposes, no dictionaries explicitly define deserted island - if you want to know what that means, you have to look up each word separately and work it out yourself.

I hope that's enough to show desert island is a well-known collocation with numerous specific connotations to most Anglophones (notably including Robinson Crusoe, Desert Island Discs, and humorous postcards). Whereas deserted island is just two words that might occur consecutively.

In short, whether there's any etymological justification for it or not, most Anglophones understand desert island as tropical, sandy-beached, [probably] without permanent inhabitants, etc., etc. They certainly wouldn't use that collocation in respect of uninhabited islands of Scotland, for example (which they might well be prepared to call deserted islands).

TL;DR: desert island evokes arid, sandy, desert-like, deserted evokes uninhabited, abandoned.
Dictionaries can only take you so far. In this case, established cultural associations are crucial.

Could one say, then, that 'deserted island' is just a collocation, whereas 'desert island' is a collocation which has evolved into a noun phrase, a compound noun?
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user58319Jan 5 '14 at 15:44

@user58319: Pretty much, yes. I only posted my answer because I thought it was hopelessly misleading of the existing answer to say they're often used as if they were synonyms. The reality is that as a pair of words, "desert island" has two or three well-known and closely-connected meanings that are only distantly related to the far less common "alternative"
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FumbleFingersJan 5 '14 at 16:05

...you probably can't see it, because it you need high rep to see deleted answers, but another user posted and then deleted an answer including the text It comes from 'desert' in the sense of sandy expanse of nothingness. So a desert island is a sandy island with nothing (or very little) alive.. He probably only deleted it because he was intimidated by people throwing weighty dictionaries in his direction. But if he hadn't, I'd have just upvoted @Rory Alsop's answer and left it at that.
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FumbleFingersJan 5 '14 at 16:14

@FumbleFingers, or maybe he deleted it because he realised it’s not etymologically correct. Desert (the noun) originally just meant ‘abandoned/deserted/desolate, remote, wilderness of a place’; it is etymologically related to the verb ‘to desert’. Up until the 19th century, desert was also used of places that are not arid or sandy, and a desert island was originally just a simple collocation of the two words. Only when desert became narrowed down to a specifically dry, sandy, arid area did desert island really become a compound with a non-transparent meaning.
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Janus Bahs JacquetMay 15 '14 at 11:53

@Janus: I'm not too sure about your non-transparent meaning there. I'm not really interested in the etymology as such here, but it seems obvious to me that most people today would strongly associate desert island with sandy, arid - the uninhabited connotations arise almost incidentally from that. I'm astonished that the overwhelmingly-favoured answer here (75% of which is irrelevant digression) makes no mention of this association, and simply defines the "standard" form as an island which has never been inhabited (as opposed to one which has been inhabited and then abandoned).
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FumbleFingersMay 15 '14 at 13:49

They are often used as if they were synonyms… But, "a desert island" is "an island which has never been inhabited, which is and has always been uninhabited", whereas "a deserted island" should mean "an island which once was inhabited but whose inhabitants left for one reason or another, whose inhabitants deserted it".

Although I am sure that this should, this normative attitude towards languages, is definitely not everybody's cup of tea! And tea-totallers are right in the end…

This reminds me of a game show I once saw on (French) TV: the people on set were given a phrase like 'not to beat about the bush' and given four different meanings the phrase might have.

The presenters had asked a hundred people in the street to choose between the different meanings of the phrase. To win the game, the people on set did not have to give the right answer, but the answer with the highest percentage.

At first this sounded preposterous to me… but then it dawned on me that this is exactly the way languages work in the long run: a mistake taken up by a majority of people ends up not being one!

An example: the sour cherry, in French, is l'agriotte from aigre, meaning 'sour' ('eager' belongs to the same word family as aigre). A lot of people mistook this for la griotte… which is the correct form today!

Similarly, when enough people have used nonnamous instead of anonymous, when Greek is Greek to enough of us, and when humourous has become plain, nonnamous might become correct!

I like your answer, but it would be better with links. ;)
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medicaJan 5 '14 at 10:07

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@Susan I had a look at the definitions of the noun phrase 'desert island' and the adjective 'deserted' in the Longman Dictionary of Contemporary English. I am currently discussing 'Lord of the Flies' with my students and repeatedly heard 'deserted island' instead of 'desert island', which made me wonder whether they were synonyms or not.
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user58319Jan 5 '14 at 10:40

Very interesting, and an excellent example.
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medicaJan 5 '14 at 10:42

So if I am stranded on a desert island, it is no longer a desert island since it is now inhabited. And when I am rescued it will then become a deserted island, never again to be a desert island?
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bibJan 5 '14 at 17:30

I live in the desert heat island of Phoenix, Arizona. It doesn't seem very tropical or sandy-beached, but as far as "without permanent inhabitants", we do get a lot of Snow Birds in the winter from Chicago and Canada.

I think it is one of those phrases which has gotten misused with time. Deserted island became desert island because people left off the "ed". There us no basis of comparison for an island with a desert because the word desert has an explicit meaning which is unrelated to an island unless the topography is like that of a desert and the fact that it is surrounded by water dismisses some if that slight similarity even.

To a British-English speaker, like me, the point of a "desert island" is that it has the characteristics of a desert as well as an island. The stereotypical desert island is hot, sandy and has coconut palms growing on it.
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Simon BMay 15 '14 at 12:03

Deserted implies uninhabited, not necessarily once, but no longer, inhabited. Desert, as in "desert island," is an otherwise obsolete form of deserted. Compare "For oh, for oh, the hobby horse is forgot," (instead of "forgotten.")

That use of forgot is completely different from desert. Forgot is just an alternative past participle of forget, while desert /ˈdɛzəɹt/ is synchronically quite unrelated to desert /dɪˈzəɹt/.
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Janus Bahs JacquetMay 15 '14 at 11:48

Desert: deserted, forsaken (archaic). The source is the Merriam Webster New International Dictionary, 2nd Ed. "Desert" is an archaic past participle, as is "forgot."
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A.BerckMay 15 '14 at 12:21

That doesn’t mean it’s an obsolete form of deserted, just that it’s an archaic adjective with the same meaning. Deserted is the past participle of the verb; desert is not, it is just an adjective. [After your edit:] The online version of Merriam Webster (which is all I have access to) has no mention of it as a participle, and while the OED claims that it was “sometimes” used as one, it has no citations to back it up.
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Janus Bahs JacquetMay 15 '14 at 12:24